Leetcode Restore IP Addresses

vector<string> restoreIpAddresses(string s) {
    vector<string> res;
    int n = s.length();
    if(s=="" || n > 12) return res;
    for(int i = 0 ; i < 3; ++ i){
        for(int j = i+1; j < min(i+4,n-2); ++ j){
            for(int k = j+1; k < n-1; ++ k){
                string a = s.substr(0,i+1);   int aa = stoi(a); 
                string b = s.substr(i+1,j-i); int bb = stoi(b);
                string c = s.substr(j+1,k-j); int cc = stoi(c);
                string d = s.substr(k+1);     int dd = stoi(d);
                if(aa >=0 && aa <= 255 && bb>=0 && bb <=255 && cc >= 0 && cc <=255 && dd >= 0 && dd<=255){
                    string str_a = to_string(aa), str_b = to_string(bb), str_c = to_string(cc), str_d = to_string(dd);
                    if(str_a.length() == a.length() && str_b.length() == b.length() && str_c.length() == c.length() && str_d.length() == d.length() ){
                    string str = str_a+"."+str_b+"."+str_c+"."+str_d;
                    res.push_back(str);
                    }
                }
            }
        }
    }
    return res;
}